Glass substrates for magnetic media and magnetic media based on such glass substrates

ABSTRACT

Described are substrates for magnetic media which have excellent surface properties as prepared directly from a slot drawdown glass sheet forming process. The substrate blanks cut directly from the glass sheet, as drawn, require little or no surface treatment for use as substrates for magnetic media, particularly hard disk drives. Also described are methods for preparing such substrates and methods for preparing magnetic media therefrom. Further described are the substrates and magnetic media prepared by such processes.

[0001] Described are glass substrates for magnetic media, for example,hard disk drives. Also described are methods for making such substrates,methods for making a magnetic medium from such substrates and theresulting magnetic media.

BACKGROUND OF THE INVENTION

[0002] The potential for use of glass substrates for magnetic media,particularly hard disk drives, has been known for some years. Advantagesof a glass substrate over the most currently used aluminum substratesinclude improved shock resistance and higher stability. For example, theimproved shock resistance lessens the possibility of “head slap” damageto the drive which can be a problem with metal substrates and theimproved stability results in disks having less flutter when driven athigh speeds and/or in thinner disks which still retain adequatestability. Additionally, a glass substrate with a highly smooth surfacecan provide a disk which can be used with the head closer to the disk,resulting in more data storage.

[0003] However, the practical applicability of glass substrates has beenhindered by the need for significant processing steps to provide asubstrate with suitable surface properties from direct pressings or fromglass sheets, i.e., the glass material directly from the apparatus bywhich it is formed into a sheet. It has been known to prepare sheets ofglass by a number of processes, including downdraw, float glass, updrawand rolling methods. As for preparation of glass sheets for use asmagnetic media substrates, one type of downdraw method, known asfusion-type downdraw, has been disclosed; see Kitayama, U.S. Pat. No.5,725,625. However, glass sheets produced by this fusion downdraw methodstill have insufficient surface properties, i.e., the required flatness,waviness, thickness variation and surface roughness, for direct use inpreparing substrates for magnetic media. Thus, as disclosed in Kitayamafor example, it was necessary to subject the glass sheet to asignificant flattening step. In fact, the Kitayama invention is mainlydirected to methods for flattening the glass sheet prepared by thefusion downdraw process. Further, the disks cut from the flattened glasssheet in Kitayama (i.e., the substrate blanks) require significantsurface treatment to provide a substrate with sufficient surfaceproperties for use as a magnetic media substrate, particularly a harddisk drive substrate. Thus, in Example 2-1 of Kitayama, the blank cutfrom the flattened glass sheet still requires a course polishing, alapping and two additional polishing steps to provide a substrate withadequate surface properties.

[0004] Sheets of glass prepared by float glass methods also do notresult in an adequate combination of surface properties. Combination ofsurface properties, and the float glass method has the additionaldrawback of requiring a processing step of leaching the glass ofcontaminants inherent in the float glass method and/or of contaminantsbeing left on the glass surface.

[0005] All of these additional processing steps require significant timeand resources. They also require starting with a glass sheet ofsufficient thickness such that, after all the surface treatments whichremove material are completed, the resulting magnetic media substratehas the proper thickness. Thus, an expense is added to the method by thenecessity of forming a blank of higher thickness, e.g., the expense ofmelting and processing a greater amount of glass material per substrateblank.

[0006] The known methods for providing glass substrates for magneticmedia are deficient in failing to provide a practically applicableprocess wherein a substrate having the required superior surfaceproperties can be cut directly from a glass sheet, as formed, such thatlittle or no surface treatment is required for use in preparing magneticmedia, particularly hard disk drives.

SUMMARY OF THE INVENTION

[0007] At least in part to overcome the deficiencies in the art, it isan object of this invention to provide substrates for magnetic media,particularly hard disk drives, cut from a glass sheet, as drawn, whichrequire little or no surface treatment. Other objects include methodsfor preparing such substrates, methods for preparing magnetic mediacomprising such substrates and the resulting magnetic media products ofsuch processes. Upon further study of the specification and appendedclaims, further objects and advantages of this invention will becomeapparent to those skilled in the art.

[0008] Accordingly, the invention includes:

[0009] A substrate for a magnetic medium prepared from a glass sheet orglass ribbon drawn from a slot downdraw process, particularly whereinthe glass sheet has the following properties, as drawn from the slotdowndraw process without any subsequent surface treatment:

[0010] flatness≦25 microns, particularly≦10 microns,

[0011] waviness less than 100 angstroms, particularly less than 40angstroms,

[0012] thickness variation±20 microns, particularly±15 microns, and

[0013] surface roughness, less than 10 angstroms, particularly less than5 angstroms.

[0014] A substrate for a magnetic medium wherein the substrate surfaceis provided with adequate surface properties solely by touch polishingand cleaning of the glass sheet as drawn from a slot downdraw process.

[0015] A substrate as described above which is for a hard disk drive,particularly wherein the substrate is in disk shape and has a magneticlayer thereon.

[0016] A method for preparing a glass magnetic media substrate whichcomprises:

[0017] drawing a glass sheet from a slot downdraw process, and

[0018] cutting said glass magnetic media substrate from the glass sheet.

[0019] The above method wherein the substrate has a flatness≦25 microns,waviness less than 100 angstroms, thickness variation±20 microns andsurface roughness, less than 10 angstroms, prior to any surfacetreatment subsequent to drawing from the slot downdraw process.

[0020] The above method wherein the only treatment which results inremoval of glass material from the substrate surface is touch polishingand cleaning, preferably where the touch polishing decreases the totalthickness of the substrate by a maximum of 0.02 mm, preferably a maximumof 0.01 mm.

[0021] A magnetic medium comprising a glass substrate prepared by amethod as described above and a magnetic layer, particularly wherein themagnetic medium is a hard disk drive.

[0022] A method for preparing a hard disk drive having a glass substratewhich comprises:

[0023] drawing a glass sheet from a slot downdraw process,

[0024] cutting a disk from the glass sheet,

[0025] touch polishing and cleaning the disk, and

[0026] applying a magnetic layer to the disk surface.

[0027] The raw material glass used in the slot downdraw process toprepare a glass sheet for use according to the invention can be selectedfrom a wide variety of materials. For use as a substrate for a magneticmedium, however, it is preferred that the glass have the followingproperties:

[0028] a. chemical temperability

[0029] b. expansion coefficient from 35 to 100×10⁻⁷ C⁻¹

[0030] c. Young's modulus prior to tempering of more than 65 Mpa

[0031] d. density (g/cm³) of from 2.3 to 2.7

[0032] e. hardness of more than 500 HK (Knoops Hardness, as per ISO 9385procedure)

[0033] It is also desirable that the glass possess only smalltemperature versus viscosity variations within the forming region inorder to attain an optimum process achieving desired dimensionalcharacteristics. This is because control of the flow of the glassthrough the slot during the downdraw must be uniform in order to providea glass sheet of specified thickness and properties.

[0034] Non-limiting examples of useful glass include aluminosilicate,calcium silicate, zinc silicate and borosilicate glasses, theborosilicates being particularly preferred. Glass compositions should beformulated to provide a functionally insignificant alkali interactionbetween the glass su rate and the magnetic layer(s) of the magneticmedia. Examples of useful glass compositions typically contain, on amole% based on the oxide, 60-75% SiO₂ 0-12% B₂O₃, 0-17% Al₂O₃, 6-13%Na₂O, 3-8% K₂O, 0-10% ZnO, 0-4% TiO₂, 0-5% MgO, 0-10% CaO, and 0-1% BaO.Minor amounts of As₂O₃, Sb₂O₃ or other processing aids may beincorporated for refining purposes. Specific examples of useful glassmaterials are described in the following Table 1, together with thematerial properties thereof. TABLE 1 Compositions Suitable for theDowndraw Process Glasstype Classification Oxide Alumino- Calcium- Zinc-Boro- Boro- Calcium- Mole % Glass Spec Silicate Silicate SilicateSilicate Silicate Silicate SiO2 61.7 71.46 67.13 69.77 74.44 73.73 B2O30.93 7.89 10.52 Al2O3 16.8 1.31 1.78 2.69 Na2O 12.2 8.07 9.07 6.75 9.058.17 K2O 4.1 6.28 6.33 4.79 4.87 5.65 ZnO 2.77 9.60 4.74 3.34 TiO2 0.80.32 0.49 3.28 0.16 0.20 MgO 3.7 4.50 CaO 0.2 9.67 0.02 0.23 7.92 BaO0.67 0.88 As2O3 0.5 0.08 0.09 Sb2O3 0.04 0.06 0.09 0.07 0.13 PropertiesChemical Yes Yes Yes Yes Yes Yes Yes Temperability Exp. Coef.  35-100 8696 96 72 83 94 {circumflex over ( )}10-7C-1 Youngs Modulus  >65 73 72 6973 82 72 Mpa prior to chem. Tempering Density 2.3-2.7 2.5 2.5 2.6 2.52.5 2.55 gm/cm3 Hardness HK >500 595 511 n/a 590 610 540

[0035] Additionally useful glass raw materials include, for example,those described in U.S. Pat. No. 4,148,661, the disclosure of which isincorporated herein by reference.

[0036] As discussed in the background section above, fusion downdrawmethods were taught in the art (Kitayama) for preparing substrates formagnetic media, however, the fusion downdraw process was deficient inproviding a glass sheet of adequate flatness and surface properties. Thefusion downdraw process uses a drawing tank with a channel for acceptingmolten glass raw material where the channel has weirs open at the topalong the length of the channel on both sides so that, when the channelfills, the molten glass overflows the weirs and flows by gravity downthe outside surfaces of the drawing tank. These outside surfaces extenddown and inwardly so that they join at an edge below the drawing tank.The two flowing glass surfaces join at this edge to form a singleflowing sheet, i.e., they fuse. The general features of the fusiondowndraw process are described, for example, in U.S. Pat. Nos. 3,338,696and 3,862,609 the disclosures of which are incorporated herein byreference. The references indicate that the advantage of the fusiondowndraw method is that since the two films flowing over the channelfuse together, neither outside surface of the resulting sheet comes intocontact with any part of the apparatus and, thus, its surface propertiesare not affected by such contact.

[0037] The slot downdraw method, schematically shown in FIG. 1 +L, isdistinct from the fusion downdraw method. According to the process, themolten raw material glass (1) is provided to a drawing tank (2) which isheated by resistance heaters (3). The bottom of the drawing tank has anopen slot with a nozzle (4) extending the length of the slot. The glassflows through the slot/nozzle and is downdrawn as a sheet (5)therethrough and then through an annealing region (6) by a drawingmachine into a continuous sheet. Compared to the fusion downdrawprocess, the slot downdraw process has the advantage of providing athinner sheet of glass having the desired properties because only asingle sheet is drawn through the slot rather than two sheets beingfused by the fusion downdraw process. In particular, the slot downdrawprocess advantageously provides a substrate with a thickness of from0.03 to 1.9 mm, although a thickness of from 0.8 to 1.1 mm is preferablyapplicable for hard disk drive substrates. Applicants have discoveredthat, although the glass obviously contacts the sides of theslot/nozzle, the process will provide a glass sheet of excellent surfaceproperties for magnetic media substrate use.

[0038] To obtain a suitable substrate according to the invention, theslot downdraw process must include tight control over the viscosity ofthe glass during the different phases of the sheet formation; see theschematic of FIG. 2 +L. Thus, the ability to form a sheet and providethe desired surface properties is affected by the viscosity of the glassmelt provided in the drawing tank, the viscosity of the glass wheresheet forming takes place in the range between the forming point and thesoftening point of the glass and especially of the viscosity duringannealing.

[0039] It has been discovered that the slot downdraw process forpreparing flat panel display substrates as described by D. Boettger,Glass Technology International, Year IX, no. 4 (July-August 1998),provides useful teachings for preparing a glass sheet useful formagnetic media substrates according to the invention; it is, thus,incorporated herein by reference. Boettger teaches the desirability ofproviding the molten and refined glass as uniform as possible and fullymixed to the drawing tank. Means for such are exemplified in Boettger,for example, by using the platinum or platinum alloy technologydescribed therein.

[0040] Also as discussed in Boettger, the geometry characteristics ofboth the drawing tank and the nozzle for the slot will affect theuniformity of the drawn product. The geometry and height of the nozzleare provided such that uniform glass thickness over the whole width ofthe nozzle and thus in the resulting glass sheet is achieved. The cornerareas of the slot and nozzle are particularly important. If the cornerareas are too wide, the out-flowing glass melt is too hot and will flowfaster than the melt towards the center of the slot/nozzle. If thecorner areas are too narrow, the out-flowing glass melt will cool toofast and flow slower than the melt towards the center. In either ofthese events, stresses will occur in the glass which will distort itsproperties.

[0041] Further, the viscosity can be affected by heating. Thus, both thedrawing tank and the nozzle are preferably capable of being adjustablyheated in different zones, preferably by direct resistance heating.

[0042] It is also of importance to control the temperature of the drawnglass sheet after it leaves the nozzle. After leaving the nozzle,different forces influence the conformity of the glass sheet, e.g., thedownwards directed tensile force generated by the drawing machine,earth's gravity and the surface tension. Thus, it is preferable to havea pre-annealing temperature adjustment of the glass sheet after itleaves the nozzle. Subsequently, the more conventional annealing stepcan be conducted. The annealer is necessary to quickly cool the glasssheet in a controlled and uniform manner. Additionally, the viscosity ofthe drawn glass sheet has to be increased such that it holds its desiredshape despite the above-discussed forces being applied to it. Thedrawing machine used can be from those conventionally known for thinsheet applications.

[0043] According to invention, a glass sheet is obtained directly fromthe above-described slot downdraw process which has superior surfaceproperties, preferably:

[0044] flatness≦25 microns, particularly≦10 microns,

[0045] waviness less than 100 angstroms, particularly less than 40angstroms,

[0046] thickness variation±20 microns, particularly±15 microns, and

[0047] surface roughness, less than 10 angstroms, particularly less than5 angstroms.

[0048] These values are determinable according to known methods, such asstandards for measuring surface properties of flat panel displaysubstrates, as described in SEMI D15-1296 by SEMI (1996). The basis forthe flatness, waviness and roughness properties is demonstrated in FIG.3 wherein it seen that the actual traced profile has components offlatness (i.e., variation over the whole scale of the surface), waviness(i.e., variation from complete smoothness on an intermediate scale) andsurface roughness (i.e., variation from complete smoothness on thesmallest scale).

[0049] With properties within the values described above, the glasssheet can be cut into blanks suitable for magnetic media with little orno surface treatment. For instance, the glass sheet can provide a blankfor magnetic media which only requires touch polishing and cleaningbefore application of a magnetic layer. Because the glass sheet alreadyhas superior surface properties from the downdraw process, additionalsurface treatment steps can be avoided, e.g., flattening, grinding,lapping (sand blasting) and/or heavy polishing steps. Thereby, a greatsavings in time and elimination of additional apparatus is achieved.This is particularly advantageous to magnetic media manufacturers whopurchase blanks and prepare magnetic media, particularly hard diskdrives, therefrom. Further, there is a savings associated with nothaving to form as thick of an initial glass sheet because less materialis removed by the above-mentioned surface treating steps to form thefinal product.

[0050] The invention provides advantageous economies in the method ofpreparing glass magnetic media substrates. As discussed, the known glasssubstrates were prepared by processes involving forming a glass sheetand subjecting the glass sheet to various flattening and surfacemodifying steps to provide a substrate with adequate properties formagnetic media. Substrates prepared according to applicants'invention donot require flattening and/or heavy surface treatment.

[0051] As stated above, blanks for magnetic media can be prepared fromthe glass sheet as drawn from the downdraw process with little or nofurther surface treatment. The blanks are prepared from the glass sheetby steps known in the art. For example, the glass sheet is cut intouniformly sized sheets which are stacked together with an interleavinglayer between each sheet, preferably adhered to each sheet. The stacksof glass sheets with interleaving layers are then drilled to cut theoutside diameter of the disk and the inside diameter of the center holeof the disk. Each sheet can provide multiple disks which number of disksis multiplied by the number of sheets in the stack to provide the totalnumber of disks provided per one operation of drilling a stack.Alternative to drilling, disks may be laser cut from individual sheetsto prefinished dimensions. The invention is preferably applied to makingdisks of any size currently popular in the industry. Current popularsizes include what are termed disks for 5.25, 3.5, 3.0, 2.5 and 1.8 inchdrives. The actual dimensions of the disks for such drives are asfollows: 5.25 (OD 130 mm, ID 40 mm), 3.5 (OD 95 mm, ID 25 mm), 3.0 (OD84 mm, ID 25 mm), 2.5 (OD 65 mm, ID 20 mm) and 1.8 (OD 48 mm, ID 12 mm).However, the invention is not limited thereby if different sizing isrequired. After drilling, the disks are provided in cylinder stacks withthe adhesive interleaving layer holding the cylinder stack of multipledisks together. The inside diameter and edges and outside diameter edgesare preferably then polished while being maintained in the cylinderstack and then cleaned to remove polishing compound. The resultingcylinder stacks of blanks are ready in this form to ship to customersand provide the distinct advantage that the customer need only separatethe blanks from the cylinder stacks and subject them merely to touchpolishing and cleaning to provide a substrate for application of amagnetic layer to prepare a magnetic medium, particularly a hard diskdrive.

[0052] The touch polishing can be conducted by known methods. Preferredare the known methods for chemical-mechanical polishing (CMP) using acerium oxide aqueous slurry polishing compound with de-ionized waterunder pressure applied by a plastic (e.g., polyvinyl alcohol) pad. Thetouch polishing or kiss polishing results in only minimal materialremoval, preferably a reduction in total thickness of the disk of 0.02mm, preferably 0.01 mm, at most. For example, a disk of 1.0 mm thick canbe provided by touch polishing of a 1.01 mm thick blank to remove only0.01 mm thickness of material from the blank surface.

[0053] Either before or after the touch polishing is conducted, it ispreferable to subject the blanks to chemical tempering forstrengthening. The tempering is conducted by known methods, for example,by treating the heated blanks with a heated solution containing ions ofa larger ionic radius to replace ions in the glass surface. Forinstance, the tempering solution may contain sodium and potassium ionswhich will replace, respectively, lithium and sodium ions at the surfaceof the glass, thus, placing the surface in compression.

[0054] The blanks, preferably chemically tempered, touch polished andcleaned, are suitable substrates for application of a magnetic layer toprovide a magnetic medium, particularly a hard disk drive. The magneticlayer can be provided, for example, by known methods. For instance, thisis typically accomplished by sputtering of one or more magnetic layers,optionally after sputtering of one or more underlayers, and a protectivelayer over top of the magnetic layer(s). Glass substrates provide anadditional advantage over aluminum substrates here because glass canaccommodate a higher range of temperatures without losing its shape.Thus, a greater flexibility is provided to use sputtering steps whichmay require greater temperatures.

[0055] The invention thus provides a more economical process forpreparing glass substrates for magnetic media which avoids disadvantagesencountered in the art. Particularly, the invention provides a methodfor preparing glass magnetic media substrates which requiresignificantly less processing subsequent to drawing of the glass sheetto prepare magnetic media. Also, the invention provides the glasssubstrates prepared by this process, and magnetic media based on suchsubstrates, which have advantages over the previous glass magnetic mediasubstrates and media therefrom because the substrates are not subject tothe subsequent processing steps which may affect their properties.

[0056] The entire disclosure of all applications, patents andpublications, cited above and below, is hereby incorporated byreference.

[0057] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

We claim:
 1. A substrate for a magnetic medium prepared from a glasssheet drawn from a slot downdraw process.
 2. The substrate of claim 1,wherein the glass sheet has the following properties, as drawn from theslot downdraw process without any subsequent surface treatment:flatness≦25 microns, waviness less than 100 angstroms, thicknessvariation±20 microns, and surface roughness, less than 10 angstroms. 3.The substrate of claim 1, wherein the glass sheet has the followingproperties, as drawn from the slot downdraw process without anysubsequent surface treatment: flatness≦10 microns, waviness less than 40angstroms, thickness variation±15 microns, and surface roughness, lessthan 5 angstroms.
 4. The substrate of claim 1, wherein the substratesurface is provided with adequate surface properties solely by touchpolishing and cleaning of said glass sheet as drawn from the slotdowndraw process.
 5. The substrate of claim 1, wherein the magneticmedium is a hard disk drive.
 6. A hard disk drive which comprises asubstrate according to claim 1 in disk shape and a magnetic layer.
 7. Amethod for preparing a glass magnetic media substrate which comprises:drawing a glass sheet from a slot downdraw process, and cutting saidglass magnetic media substrate from the glass sheet.
 8. The method ofclaim 7, wherein the substrate has the following properties prior to anysurface treatment subsequent to drawing from the slot downdraw process:flatness≦25 microns, waviness less than 100 angstroms, thicknessvariation±20 microns, and surface roughness, less than 10 angstroms. 9.The method of claim 7, wherein the substrate has the followingproperties prior to any surface treatment subsequent to drawing from theslot downdraw process: flatness≦10 microns, waviness less than 40angstroms, thickness variation±15 microns, and surface roughness, lessthan 5 angstroms.
 10. The method of claim 7, wherein the only treatmentwhich results in removal of glass material from the substrate surface istouch polishing and cleaning.
 11. The method of claim 10, wherein thetouch polishing decreases the thickness of the substrate by a maximum of0.02 mm.
 12. The method of claim 10, wherein the touch polishingdecreases the thickness of the substrate by a maximum of 0.01 mm. 13.The method of claim 7, wherein the glass magnetic media substrate is cutfrom the glass sheet in the form of a disk and is a substrate for a harddisk drive.
 14. The method of claim 7, wherein a plurality of sheets ofglass are arranged as a stack together with an adhesive interleavinglayer in between each sheet and a plurality of substrates are cut fromthe stack in a single cutting step.
 15. The method of claim 7, whereinthe glass magnetic media substrate has a thickness from 0.8 to 1.1 mm.16. The method of claim 7, wherein the glass magnetic media substratehas a thickness from 0.03 to 1.9 mm.
 17. A magnetic medium comprising aglass substrate prepared by the method of claim 7 and a magnetic layerthereon.
 18. A magnetic medium comprising a glass substrate prepared bythe method of claim 10 and a magnetic layer thereon.
 19. The magneticmedium of claim 17, wherein the magnetic medium is a hard disk drive.20. A method for preparing a hard disk drive having a glass substratewhich comprises: drawing a glass sheet from a slot downdraw process,cutting a disk from the glass sheet, touch polishing and cleaning thedisk, and applying a magnetic layer to the disk surface.
 21. The methodof claim 20, wherein the only treatment which results in removal ofglass material from the disk surface after drawing of the glass sheet istouch polishing and cleaning.
 22. The method of claim 17, wherein thetouch polishing decreases the thickness of the disk by a maximum of 0.02mm.
 23. The method of claim 17, wherein the touch polishing decreasesthe thickness of the disk by a maximum of 0.01 mm.